It has been conventional practice to prepare various textile reinforcing fibers to be used in contact with rubber formulations by pretreating them with a mixture of a rubber latex and a phenolformaldehyde resin in which the phenol has almost always been resorcinol. This is the so-called "RFL" (resorcinol-formaldehyde-latex) method. Another method commonly used is to generate the resin in situ (in the vulcanized rubber/textile matrix) by incorporating in the rubber a formaldehyde (or methylene) donor compound, for example, hexamethylenetetramine or hexamethoxymethylmelamine and a formaldehyde (or methylene) acceptor compound, for example, a dihydroxybenzene compound such as resorcinol. The in situ method hats been found to be particularly effective where the reinforcing material is brass-coated steel wire since pretreatment of the wire with the RFL system has been observed to be largely ineffective.
The in situ formed resins have been observed to enhance adhesion between the rubber and the reinforcing material. This enhancement of adhesion is by a mechanism that is not completely understood. The in situ method which entails compounding a vulcanizable rubber stock with the phenol-formaldehyde components is known in the art. The most commonly employed methylene acceptor is resorcinol, while the more commonly employed methylene donors are the N-substituted oxymethylmelamines. The resin is formed in situ during vulcanization of the rubber creating a bond between the fiber and the rubber irrespective of whether the fiber has been pretreated or not.
The rubber industry utilizes resorcinol in conjunction with methylene donors to form resins which are used in rubber compounds to enhance filamentary reinforcement and to improve the physical properties, including aged physical properties, or tires and other compounded rubber formulations. These resins, whether preformed or formed in situ, can increase the dynamic rubber stiffness, tire treadwear, interfacial adhesion and fabric adhesion to the rubber compound; while desirably maintaining the values of and hopefully reducing the rolling resistance and heat buildup of a tire.
With the constant demand on quality, producers of reinforced vulcanizates are under pressure to produce more products able to withstand greater fatigue and longer-term service. Therefore, any achievements in improving the interfacial adhesion between the reinforcement and rubber environment is needed.